Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

Identifying Virgin Plastics

Despite ample data on the properties of various polymers, it is easy to understand that most end-users rely on the word of the supplier that the polymer they are paying for is the polymer they are getting.

Because many of the properties are inherently difficult to test, one would need to send the material to a lab for identification. And because the methods of identification are sometimes complex and require many types of tests, this can end up being an expensive affair. A client may be willing to undertake this expense one time, but if a component is supplied regularly, it would be cumbersome to test the materials each time a new lot is received. Hence, for the most part, clients accept the material test certificates (MTC) as provided by the supplier and trust that the parts supplied are from the lot corresponding with the MTC.

In our own experience, we have come across many instances of clients claiming they are using a certain polymer when in reality, the material they are being supplied is a cheaper variant of the polymer they think they are using.

A few examples of these are highlighted in the table below:

Polymer Substitute Price difference
PEEK PEK, PAEK 2X
PTFE Polypropylene, Polyethylene 4-5X
UHMWPE LDPE, HDPE 2-3X
PCTFE PTFE, Polypropylene 10-20X
FEP Polypropylene, Polyethylene 40-50X
PA66 PA6 1.5-2X
PFA Polypropylene, Polyethylene 40-50X

 

In most cases, the criteria for this substitution is clearly price. We receive many samples from potential clients claiming to be either PEEK or PTFE. In some cases, a lab test is not even needed as it is visually obvious they are using another polymer.

In a few cases, the non-availability or the non-processability of the polymer leads to suppliers opting for substitutes. For example, the inability of UHMWPE to be easily injection moulded leads some processors to use LDPE or HDPE instead. Visually, it is difficult to tell these polymers apart, so the client accepts the alternate material without question.

Obviously, the performance of these materials cannot match up to the polymer originally chosen for the application.

In one case, we received samples from a client claiming they were PEEK and enquiring as to why they should have failed in his application. The part was a ball valve seat, procured from another vendor and had deformed after only a few months of performance. When we explained that the part was PEK, the client insisted that his supplier was giving him PEEK. When the part was sent to the lab and it was confirmed that the part was PEK, the client asked us to supply him the same part, but with virgin PEEK. When we explained that the price would be nearly double, they refused to accept, asking us to match the price they were already getting. Eventually, they returned to their original supplier, even though they knew the material being supplied was inferior. The commercial impact of using virgin PEEK was too high for them and they preferred using the cheaper variant and dealing with the rejections that came with this.

In an attempt to make the identification of certain polymers more transparent, we created the above infographic. Using this, basic tests can be performed to ensure that the polymer is as committed.

Metal Replacement with PEEK

There is virtually no industry that does not strive for reductions in weight of the final product. In some cases, higher weight implies higher costs of the product itself; in some, weight reduction leads to improved efficiencies of the equipment.

Advancements in polymer science over the past few decades have allowed us to develop formulations that come close to mimicking the strengths offered by metals. While it is fair to say that on average, the strength of a polymer lags significantly against the toughness of metals, there are several applications where the polymer offers more than adequate strength, whilst bringing a host of other benefits that make it a more viable option.

We were recently approached by a client inquiring whether PEEK parts could replace the existing metal parts being used. This led to some research as to the compositions that would be needed to augment the tensile and yield strengths of PEEK beyond what it offered in its natural (virgin) state.

PEEK is a crystalline material that ranks as having amongst the highest tensile strengths within the polymer space. It is also a light-weight polymer, which makes it highly sought after in applications where weight reduction is crucial. In addition to this, the introduction of certain fillers allows the strength of PEEK to be enhanced significantly.

  SS 304 Virgin PEEK Reinforced PEEK Units
Ultimate Tensile Strength 515 95 586 MPa
Specific Gravity 8 1.3 1.52  
Youngs Modulus 190 3.65 57 GPa
Elongation at Break 55 60 1.6 %
Service Temperature 900 300 300 °C
         
Cost Index (SS=1) 1 32 20  
         
Adjusted Cost over SS 5.2 3.8  

 

The table above shows the improvements that can be made when adding reinforcements to PEEK when compared to Stainless Steel 304.

  1. It is apparent that we can exceed the tensile strength of steel, although this comes at the cost of very low elongation.
  2. The Young’s Modulus (or Tensile Modulus) of stainless steel remains significantly higher than that of reinforced PEEK. When combined with the data on elongation, this tells us that steel still has a higher tolerance to loads and will deform when the loads increase. Reinforced PEEK, however, will most likely crack when pressures of over 57 Gpa are applied
  3. At elevated temperatures, PEEK would still lag stainless steel, or indeed most metals. However, with a temperature rating of 300°C, PEEK is still sufficiently capable of handling most industrial applications
  4. PEEK is over 30 times the cost of stainless steel. Even with reinforcement – which brings the price of PEEK lower as the fillers are not as expensive as virgin PEEK – the cost is 20 times that of steel. However, when you factor in the reduction in densities, the numbers fall to 5.2x and 3.8x respectively

    The implication of this however is that replacing steel with PEEK would bring a four-fold cost increase. Hence the decision to replace cannot be based on cost savings.
  5. In addition to the basic properties listed above, PEEK offers a range of other benefits such as lower coefficients of friction, lower thermal conductivity, higher resistances to chemicals and electrical insulation.

The above data tells us why, while PEEK can certainly match up to stainless steel in some regards, the replacement is done in only very specialised fields.

  • Aerospace

    PEEK is used extensively in this industry, replacing steel and even aluminium in many cases. Since the cost of replacement is one-time, but the benefits if weight reduction are continuous, PEEK is the preferred material of choice in aerospace.

PEEK is also a safer material to use, since its low conductivity allows for lower heat build-up. Furthermore, if a part does break off and/or release debris – the resulting damage is minimised as the material is not hard enough to jam other metallic moving parts.

  • Healthcare and Pharma

Being chemically inert, PEEK is preferred to metals in areas where chemicals may be present and the reaction to metals may not be fully understood or predictable.

Medical devices may also use PEEK as the equipment may benefit from being light weight. Certain scanning devices shun metallic parts, as they may affect the readings. Here again, PEEK is used to ensure the device is physically durable but does not have too much metal in its structure

  • Oil and Gas

PEEK is tough enough to withstand some of the harshest forces, while offering a resistance to corrosive fluids and high temperatures. In most cases, oil & gas equipment is placed deep underground, where the forces and chemicals are not fully predictable.  Oil and gas equipment frequently employs PEEK in seals and valve plates to ensure that there is no failure once the equipment is installed.

  • Precision Instrumentation

PEEK offers an elevated level of structural rigidity, while providing electrical insulation. Connector covers, transducer covers, and cable channels are made using PEEK because it does not interfere with the electrical signals being passed through the wires within the device. Again, as weight reduction is a key criterion in this industry, PEEK is preferred even though it is expensive.

  • Food processing

Several factors make PEEK the material of choice in food processing. For one, the material does not react with or impart any flavour to the food. For example – coffee machines that traditionally used aluminium valves replaced them with PEEK, because the aluminium imparts a metallic taste to the coffee. In addition to this, at high RPMs, PEEK offers minimal heat build-up. In food processing appliances, where the body may be made with a low-temperature plastic and the designs tend to be compact, it is essential that moving parts do not become so hot as to warp/damage the body of the equipment. Finally, since liquids are often present in food preparation, PEEK is preferred as it is less likely to corrode and/or cause an electrical short circuit within the device.

It is apparent that PEEK is not a low-cost alternative to metals. However, applications that look for efficiency and performance enhancements can benefit tremendously from using PEEK and not worry that its strength would not be sufficient. Since the replacement of metal with PEEK is a one-time expense in most cases, the consequent savings from the improved efficiency of the system would need to be weighed in to get a true measure of the overall cost savings.

Expanded PTFE (ePTFE) Tapes in Aerospace

Some of the most complex engineering done today revolves around the aerospace industry. Keeping a plane, satellite, or rocket airborne is a challenge involving thousands of variables, all of which keep changing during the course of operations. The equipment needs to be robust and capable of maintaining accurate, predictable performance over a long period of time. In addition to this, efficiency calls for the need to keep things light-weight.

This demanding combination of low-weight and high-strength has led to the invention of some of the most exceptional materials in recent times. Polymers such as PEEK and Polyimides have been used with reinforcement to offer metal-like levels of tensile properties, while reducing the weight by as much as 85%.

Among the most useful such materials is expanded PTFE (ePTFE).

While PTFE alone is a useful polymer in aerospace – finding applications in radar (radomes), fluid transfer (tubes, bobbins), and insulation (tapes, films) – expanded PTFE offers a range of new properties that augment the usefulness of this material further.

We look here at some of the areas of application of ePTFE tapes within the aerospace domain:

  1. Sealing function

    ePTFE has a soft, foam-like structure that easily adopts the shape of the material around it. As such, it can be sandwiched between any two harder elements to create a perfect seal instantly. ePTFE is so pliable that the seal can be created with minimal torque. The seal is able to perform even at high-pressures, meaning there is little risk that a good seal at ground level, will suddenly fail when pressures start increasing.

    In addition to the seal-ability of the material, it is also resistant to both high and low temperatures (-250°C to +250°C) and chemically inert. Again – the idea that the seal will cease to perform at extreme environmental conditions is completely mitigated.

    ePTFE tapes are usually used in areas where a permanent seal is needed and where the assembly is not likely to be disturbed frequently. It is a vital part of both build and maintenance operations, since it is available as a roll and can be very easily used on-site.

     

     

  2. Anti-squeak / anti-chafe function

    Another frequent problem with aerospace is that even with the most modern design techniques, it is impossible to fully predict which components will vibrate during operation. Vibrating parts can rub against one another causing wear outs and noise. Expanded PTFE tapes are an ideal medium to use between rubbing parts. Not only does the tape squeeze down to accommodate the exact gap between the parts, but it’s low coefficient of friction eliminates any damage that the parts can do to one another and any noise that might be created.

    ePTFE is also light-weight (the density can be as low as 0.35g/cm3), so it can be used without any worry that you would be adding load to the system.

     

     

  3. Anti-corrosion function

    In areas with heavy exposure to fuels and aviation oils, ePTFE can be used as an effective protector. Being oleophobic, ePTFE repel oils and can therefore form an effective layer over areas that would otherwise be corroded by repeated contact with oils and fuels that may splash on to it.

    The material is particularly useful in access panels and fairings where hydraulic or engine oil contamination could occur.

Apart from the areas cited about, ePTFE finds plenty of uses across both the maintenance and manufacture of the equipment. Its unique properties combine seal-ability, temperature and chemical resistance, abrasion resistance and a high strength-to-weight ratio, making it one of the most versatile materials to be used in the field of aerospace.